1. Field of the Invention
The present invention relates to an apparatus for measuring a force or load acting on a pressure member for holding a blank or workpiece in a pressing machine.
2. Discussion of the Related Art
There has been widely used a pressing machine equipped with a so-called "cushioning mechanism", which is adapted to draw a blank in the form of a metal strip or sheet. This type of pressing machine includes (a) a stationary bolster on which a lower die is fixed, (b) a movable main slide which carries an upper die fixed thereto, (c) a pressure member disposed below the main slide, (d) a plurality of cushion pins which extend through the bolster and support at their upper ends the pressure member such that the cushion pins and the pressure member are movable in the longitudinal direction of the pins, (e) a cushion pad associated with the lower ends of the cushion pins to move the cushion pins, (f) force applying means for applying a holding force to the pressure member. The force applying means has a fluid-actuated cylinder with a piston actuated by a fluid pressure for biasing the cushion pad upwards in the longitudinal direction of the cushion pins. In a drawing operation of the press, a force is transmitted to the cushion pad via the pressure member and the cushion pins, whereby the cushion pad is forced to be lowered, with a result of a retracting movement of the piston which causes an increase in the fluid pressure within the cylinder. Thus, a reaction force corresponding to the fluid pressure is applied to the pressure member so that the pressure member cooperates with the upper die to hold the metal strip under drawing. This force acting on the pressure member is hereinafter referred to as a "holding force".
An example of a press of the type discussed above is illustrated in FIG. 1, in which a drawing operation is performed by an upper die 18 and a lower die in the form of a punch 12, while a metal strip is held between a pressure member 30 in the form of a ring and the upper die 18. The press uses a fluid-actuated cylinder in the form of a pneumatic cylinder 42 for biasing a cushion pad 28 in the upward direction, by a pneumatic pressure Pa generated by the cylinder 42, so that the holding force is applied to the pressure member 30 via a plurality of cushion pins 24.
In a known pressing machine constructed as described above, the pneumatic pressure Pa is commonly adjusted or determined by a "trial-and-error" procedure conducted in trial or test operations on the specific blank to be drawn. With this procedure, the appropriate pneumatic pressure Pa which assures a drawing operation without cracking or wrinkling of the blank strip is eventually established. In this respect, it is noted that the optimum holding force suitable to permit the drawing operation as needed differs depending upon the specific die set (upper die and lower punch) and the specific pressure member. That is, different die sets are used for different blanks, and different pressure members having different weights are used with different die sets. Accordingly, the conventional procedure to determine the holding force and the pneumatic pressure Pa which assure a drawing operation on a specific blank without cracking and wrinkling requires installation of appropriate die set and pressure member on the press, and a trial drawing on the specific blank.
It is considered to replace the above manual trial-and-error procedure by an automatic procedure in which the optimum pneumatic pressure Pa for a specific blank is automatically established by suitably controlling the air pressure Pax in an air tank 44 by operating a solenoid-operated pressure control valve 46. In this case, the optimum pneumatic pressure Pax is calculated according to the following equation (1), on the basis of the following parameters: an optimum holding force Fso for assuring an adequate drawing operation on the specific blank; a weight Wr of the pressure member 30 suitable for the die set used for the blank; number n of the cushion pins 24 used with the pressure member 30; a weight Wa of the cushion pad 28; a weight of the cushion pins 24; and a pressure-receiving area Aa of the pneumatic cylinder 42. The parameters Fso, Wr and n are known from a trial or test drawing operation conducted on a test press using the relevant die set. This considered procedure eliminates the conventionally required cumbersome adjusting operation at the job site, and reduces a work load of the operator of the press, while assuring high stability in the quality of formed products, with automatic establishment of the holding force applied to the pressure member. EQU Pax=(Fso+Wa+Wr+n.WP)/Aa (1)
In the above procedure, however, the weight Wa of the cushion pad is estimated based on the nominal weight as designed, or the weight Wa of the cushion pad is actually measured when the cushion pad is not installed on the press. Therefore, the optimum pneumatic pressure in the air tank as calculated according to the above equation does not reflect influences of a sliding resistance between the cushion pad and its guide member, and a sliding resistance of the piston of the fluid-actuated cylinder. Thus, the above procedure does not permit an effective pneumatic pressure which determines the actual holding pressure, and is not satisfactory to assure practically sufficient reliability of the automatically established holding pressure Fso, and may lead to cracking or wrinkling of the formed products. Another factor which prevents the sufficient reliability of the holding force Fso is the use of the nominal or design value of the pressure-receiving area Aa of the fluid-actuated cylinder, without taking a leakage flow of the fluid into account. The nominal pressure-receiving area Aa is different from the effective value which varies with a degree of the fluid leakage of the fluid-actuated cylinder.
Further, even assuming that the holding pressure Fso acting on the pressure member is kept at an optimum level, this does not necessarily assure that the holding pressure Fso is substantially evenly or uniformly distributed throughout the entire area of the pressure member. The uneven distribution of the holding force Fso may result in cracking or wrinkling of the formed products. This problem is encountered not only in mass production of a given product on the production press, but also in a trial or test operation on a trial press which is conducted for the purpose of determining the positions of the cushion pins. In the latter case, the uneven distribution of the holding force makes it difficult to correctly determine the positions at which the cushion pins are disposed.